(1) Field of the Invention
The present invention relates to a method of fiber identification in an optical transmission network by which identification of an aimed coated fiber in an optical transmission network can be accomplished between its upstream and downstream ends during transmission of an optical signal. In this case, an optical fiber cable is composed of a plurality of the coated fibers, and a single-fiber, a multi-fiber, a ribbon-fiber etc. are commertially available for a preferred example of the coated fibers.
(2) Description of the Prior Art
A vigorous research and development program is going forward on commercial communications systems using silica-based optical fibers as transmission lines. In Japan, a large-capacity system (F-400M) as well as a medium and a small-capacity system (F-32M and F-100M) have been commercially available for interoffice trunking in public communications circuits and a 400 Mb/s capacity communications system is available today. Furthermore, in order to accommodate the versatility of services to be anticipated in the further communications systems, the application of a lightguide communications system to feeder and subscriber loops is under review.
For successful replacement of metallic cables such as coaxial cables by optical fiber cables, it is important to establish reliable maintenance technology for optical transmission network. In the maintenance of optical transmission network, a line switching technique that enables an active line to be switched to an auxiliary or spare line between interoffice subscriber terminals, and a line monitoring technique by which a lineman can check to see if an active line of interest is busy or if the line to be switched is exactly what it should be are indispensable. Optical transmission network maintenance work involving these techniques has been accomplished by removing the connector from either end of an optical fiber cable and reconnecting it to a detector or an auxiliary or spare line.
A major problem with the line monitoring and switching job is that it causes interruption of communications over the line to be monitored or switched and therefore that it cannot be executed with lines in ordinary use. Interruption of communications on account of maintenance or repair work must be avoided by all means in communication circuits, in particular, in subscriber loops. Unless this problem is solved in a satisfactory way, optical fiber communication systems cannot be applied to subscriber loops.
Monitoring of optical communication circuits is currently accomplished by the combination of Local Injection (LI) and Local Detection (LD) methods that have been used in practice for fusion splicing. An example of the local injection (LI) method is disclosed in Published Unexamined Japanese Patent Application No. 270706/1986 and an example of the local detection (LD) method is disclosed in Published Unexamined Japanese Patent Application No. 270707/1986. This technique involves bending a bundle of optical fibers in a cable at two distant locations and injecting light into the fiber at one bent portion while detecting radiated light that leaks from the fiber at the other bent portion.
The prior art of this monitoring technique is illustrated in FIG. 1 with reference to the case where, as shown in FIG. 1(a), an office 1 is connected to subscribers 2a, 2b . . . and 2n via the corresponding number of coated fibers 5a to 5n that are installed between a connector 3 on the office side and a connector 4 on the subscriber side in such a way as to provide for bidirectional communication. If an installed coated fiber 5n is to be replaced by a new one, the identification of the fiber 5n has to be established between the connector 3 on the office side and the connector 4 on the subscriber side.
This need has been conventionally met by using the combination of LI and LD methods as shown in FIG. 1(b). First, coated fibers 5a, 5b and 5c are bent in the area near the connector 4 on the subscriber side to form bent portions 25a, 25b and 25c and radiated light leaking from these portions is detected with detectors 45a, 45b and 45c, respectively (LD method). In the next place, another bend 15a is formed in the fiber 5a to be identified, and light is injected into that bent portion of the fiber from a light-emitting device 35a (LI method). Since the injected light is picked up only by the light-receiving device 45a associated with the fiber 5a, identification of the latter can be accomplished.
Coated fiber identification can also be made by bending the fibers 5a to 5n in the area near the office 1 or the subscribers 2a to 2n as indicated by dashed lines in FIG. 1(b).
The prior art method of coated fiber identification described above has the following disadvantages.
First, in order to inject an adequate power of light into the coated fiber by the LI method, the fiber must be bent with a small radius of curvature. However, if the fiber bend has a small radius of curvature, radiated light of a large power will leak from the bent portion of the fiber to which the LI method is to be applied and this can cause deterioration of the signal to be transmitted. Therefore, if the LI method is applied during transmission of an optical signal, troubles such as channel interruption will occur in optical signal communication, and in an extreme case, cracking might occur in the coated fiber.
Secondly, if a light having a power greater than a certain level is injected into a coated fiber by the LI method, it will be transmitted even to the office or subscribers resulting in an occurrence of another noise component that deteriorates the optical signal to be transmitted.